The present invention is directed to self scanned integrated displays and in particular to displays having a column driver which exhibits reduced voltage stress.
Active matrix liquid crystal displays generate images by altering the polarization of individual picture elements using a liquid crystal material. The picture elements (pixels) are arranged in rows and columns. Image data is loaded into the liquid crystal display one row at a time. The rows of pixels are sequentially scanned in order to form image frames.
Each pixel in an active matrix display includes a thin film transistor (TFT). The thin film transistor receives video data from a column driver on the display when the display row containing the pixel is selected. The TFT stores the received video data onto the capacitance of the pixel.
One material which may be used to form active matrix LCD displays is amorphous silicon. This material has the advantage that it may be fabricated at relatively low temperatures.
Because the TFTs of the pixels are fabricated from amorphous silicon. It is desirable to implement the peripheral circuitry, for example, the line scanners and column drivers using TFTs. It is difficult to design circuitry with TFTs, however, because they exhibit threshold drift. Threshold drift is a phenomenon where the gate to source voltage needed to turn on the transistor changes over time. In amorphous silicon TFTs, threshold drift occurs when a TFT is driven at a high duty cycle.
U.S. Pat. No. 5,670,979 to Huq et al. entitled xe2x80x9cDataline Drivers with Common Reference Ramp Displayxe2x80x9d discloses a column driver implemented with amorphous silicon technology. The column driver disclosed in this patent includes circuitry which adjusts the drive voltage of certain ones of the transistors to accommodate for threshold drift in these transistors. The disclosed circuit, however, drives transistors at a relatively high duty cycle, and thus undesirably reduces the expected lifetime of these transistors.
The present invention is embodied in a display device which includes a column driver having an initialization sequence in the vertical blanking interval.
According to one aspect of the invention, the display device is an active matrix display including a thin-film transistor (TFT) which is connected to drive the column of the display device. The signal used to render the column driver TFT conductive is determined in the vertical blanking interval and maintained on a capacitor in the column driver for the length of the vertical field internal.
According to another aspect of the invention, the autozero operation on the comparator of the column driver is performed during the vertical blanking interval.